For improving the stability in case of short circuit and for increasing the load change performance of, e.g., IGBTs, there has been a tendency to replace the well-known metallization using aluminum (AlSi or AlSiCu) with a thick Cu metallization in the regime of 5 μm to 20 μm. This shall provide a higher performance, enabling higher operating temperatures due to enhanced heat dissipation.
However, the application of Cu as a metallization bears some drawbacks. For example, Cu reacts with Si at temperatures as low as room temperature and results in Cu silicides. As an example, Cu3Si forms at room temperature. Hence, a barrier between the Cu metallization d the Si is required. Typically, a barrier layer of a material with a high melting point is provided, such as W, Ta, or an alloy such as TiW or TiN. Frequently, combinations of various layers employing different elements are used.
Furthermore, Cu atoms tend to diffuse into Si and can thus drastically reduce the life time of minority charge carriers. This may lead to various drawbacks, for example an undesirable enhancement of the forward voltage and the leaking current in a blocking state.
Furthermore, in a humid atmosphere, an electrochemical reaction in the presence of an electric field can lead to a discharge of Cu ions. These are produced by anodic oxidation, may start wandering due to the presence of the electrical field during operation and may under certain circumstances accumulate at the cathode, whereby Cu dendrites are formed, which is also known as electro migration. The latter process occurs primarily in the region of the edge termination, because in this region a higher electric field is present. During the drift process towards the edge region, the positively charged Cu ions may disturb the well-defined change of potential.
However, known barrier layers employing materials like TiW provide at least microscopic paths along microscopic crystal boundaries, through which Cu atoms may unwantedly diffuse from the metallization layer towards the active area, causing negative effects as described.
For these and other reasons there is a need for the present invention.